Localization of a radioactive source within a body of a subject

ABSTRACT

A computerized system for locating a device including a sensor module and a processor. A radioactive source, associated with the device, produces a signal in the form of radioactive disintegrations. The sensor module includes a radiation detector capable of receiving a signal from the source attached to the device. The sensor module produces an output signal. The processor receives output signal(s) and translates output into information relating to a position of source.

RELATED APPLICATIONS

The present application claims the benefit under 35 USC 119(e) of U.S.Provisional Application No. 60/600,725 filed on Aug. 12, 2004; entitled“Medical Navigation System Base on Differential Sensor”; 60/619,792filed on Oct. 19, 2004, entitled “Using a Catheter or Guidewire TrackingSystem . . . :”, and 60/619,897 filed on Oct. 19, 2004, entitled “Usinga Radioactive Source as the Tracked . . . :”, the disclosures of whichare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to location and tracking of a source ofionizing radiation, for example within a body of a subject.

BACKGROUND OF THE INVENTION

Existing techniques for intrabody tracking include direct video imagingusing a laparoscope; fluoroscopy (performance of the procedure undercontinuous or periodic X-Ray imaging); electromagnetic tracking, opticaltracking, computerized tomography (CT) tracking and ultrasonic imageassisted tracking. Some of these techniques explicitly avoid ionizingradiation. Those techniques which employ ionizing radiation, such asfluoroscopy and CT, require sufficient amounts of ionizing radiationthat radiation exposure for subjects and medical staff is a subject ofconcern.

Some applications which require intrabody tracking, such as cardiaccatheterization, require concurrently acquired images because the tissuethrough which the tracked medical device is being navigated movesfrequently. Other applications which require intrabody tracking, such asintracranial procedures, are more amenable to the use of pre-acquiredimages because the relevant tissue is relatively static.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the present invention relates to usingionizing radiation from a source in order to detect its position,optionally in or near the body of a subject, without production of animage. Optionally, the source is integrally formed with or attached to amedical device. Medical devices include, but are not limited to, tools,implants, navigational instruments and ducts.

In an exemplary embodiment of the invention, position of the source isdetermined by non-imaging data acquisition. For purposes of thisspecification and the accompanying claims, the phrase “non-imaging”indicates data acquired independent of an image acquisition process thatincludes the source and anatomical or other non-source features in asame image.

Optionally, position is determined using a sensor which has angularsensitivity resulting in a detectable change in output resulting fromradiation detection according to an effective angle of incidence ofradiation from the source. Greater sensitivity in effective angle ofincidence provides greater efficiency of the position determination interms of speed and accuracy. Embodiments with an angular range of lessthan ±100 milliradians, optionally less than ±50 milliradians aredisclosed. In an exemplary embodiment of the invention, greatersensitivity to effective angle of incidence can be achieved by moving aradiation detector and/or a shield.

Optionally, the source of ionizing radiation has an activity in therange of 0.01 mCi to 0.5 mCi. Optionally, the source of ionizingradiation has an activity less than 0.1 mCi. Optionally, the source ofionizing radiation has an activity of about 0.05 mCi. In an exemplaryembodiment of the invention, a radiation source which poses nosignificant health risk to a patient (i.e short term exposure) and/ormedical personnel (i.e. long term exposure) may be employed.

Optionally, the refresh rate for the location data insures that thelocational information is temporally well correlated to the actuallocation of a tracked object (e.g. medical device). Recommended refreshrates vary according to the speed at which the tracked object moves andaccording to the environment in which the tracked object moves. In anexemplary embodiment of the invention, for tracking of medical devicesthrough body parts which are more static, such as brain or digestivetract, lower refresh rates, for example 10 times/second may be adequate.In embodiments for tracking of medical devices through body parts whichmove frequently, such as the heart, higher refresh rates, for example 20times/second may be desirable. Optionally, gating to an ECG output maybe implemented so that positions from selected cardiac cycle phases areplotted.

Optionally, the RMS error of a calculated position of the source ofionizing radiation is less than 10 mm, optionally less than 5 mm,optionally less than 2 mm, optionally less than 1 mm, optionally 0.5 to0.8 mm or better.

Variables which may influence the accuracy of determined position(s)include activity of the source in DPM, the accuracy and/or response timeof radiation sensors employed for detection, and the speed of theimplanted medical device. Improvement in one or more of these variablesmay compensate for one or more other variables. Optionally, reducing thespeed of a tracked medical device may be employed to compensate forother variables. Optionally, location information is displayed in thecontext of anatomical imaging data. Optionally, relevant anatomicalfeatures are highlighted to facilitate navigation of the medical deviceby medical personnel. Optionally, determined positions may be displayedin the context of a separately acquired image.

Optionally, two or more sources may be tracked concurrently. Optionally,multi-source tracking is used in determining orientation of anasymmetric medical device. Optionally, multi-source tracking is used incoordinating activity of two or more medical devices for a medicalprocedure.

An aspect of some embodiments of the present invention relates to usinga sensor with angular sensitivity to detect a direction towards a sourceof ionizing radiation. Optionally, two or three or more directions aredetermined, either concurrently or successively, so that a position maybe determined by calculating an intersection of the directions. If threeor more directions are employed, the location may be expressed as athree dimensional position. Optionally, a direction is used to determinea plane in which the source resides.

Optionally, sensors for detection of radiation from the source achievethe desired angular sensitivity by rotation of at least a portion of thesensor about an axis through a rotation angle. For example, detectors orradiation shields may be rotated. Alternately or additionally, sensorsmay achieve the desired angular sensitivity by translational motion.

An aspect of some embodiments of the present invention relates to asensor with an angular sensitivity which causes changes in an outputsignal from at least one radiation detector in response to an effectiveangle of incidence between the detector and a source. A target value ofthe output signal is achieved at an angle indicating the directiontowards the source. The direction is optionally used to determine aplane in which the source resides.

Optionally the sensor may include more than one radiation detector, eachradiation detector having a separate output signal. Optionally, one ormore radiation shields may be employed to shield or shadow at least aportion of at least one of the radiation detectors from incidentradiation. The degree of shielding changes as deviation from the angleindicating a direction towards the source occurs and the output signalvaries according to the degree of shielding.

Optionally, multiple radiation shields are employed in concert to form acollimator. The radiation shields may be either parallel to one anotheror skewed inwards. Optionally, the multiple radiation shield, whetherparallel or skewed, may be rotated.

Optionally, the deviation from target output is 1% of the output rangeper milliradian of angular displacement away from an angle indicating adirection towards the source. Optionally deviation in output indicatesdirection of deviation as well as magnitude of deviation. According tovarious embodiments of the invention, radiation detectors and/orradiation shields may be displaced to impart angular sensitivity. Thisdisplacement may be rotational and/or translational.

An aspect of some embodiments of the present invention relates to acomputerized system for locating a medical device, optionally within abody of a subject by using angular sensitivity of a sensor module todetermine a direction. The sensor module measures incident radiation onone or more radiation detectors. Incident radiation produces an outputsignal which is translated to directional information by the system.

An aspect of some embodiments of the invention relates to association ofa source of ionizing radiation with a medical device to facilitatedetermination of a location of the device, optionally as the device isnavigated within or near a subject's body during a medical procedure.Optionally, the source of ionizing radiation has an activity in therange of 0.01 mCi to 0.5 mCi. Optionally, the source of ionizingradiation has an activity less than 0.1 mCi. Optionally, the source ofionizing radiation has an activity of about 0.05 mCi. Associationincludes integrally forming the source and the device as a single unit.Association also includes attaching the source to the device.Optionally, the source is concentrated in an area having a largestdimension less than 10 mm, optionally less than 5 mm, optionally lessthan 2.5 mm, optionally less than 1 mm.

An aspect of some embodiments of the invention relates to use of anionizing radiation source with an activity of 0.1 mCi or less as atarget for non imaging localization or tracking, optionally in a medicalcontext. The source of ionizing radiation is selected to reduce abiological effect on the patient and/or medical personnel. Thisselection involves consideration of radiation strength, radiation typeand/or amount of exposure time (e.g. time in the body for a patientundergoing a procedure). Alternatively or additionally, radiationsources which are constructed of biocompatible material and/or coatedwith biocompatible coatings may be employed.

In an exemplary embodiment of the invention, a computerized system fortracking and locating a source of ionizing radiation is provided. Thesystem comprising:

(a) at least one non-imaging sensor module comprising at least oneradiation detector, the at least one radiation detector capable ofreceiving ionizing radiation from the radiation source and producing anoutput signal; and

(b) the CPU designed and configured to receive the output signal andtranslate the output signal to directional information.

Optionally, the source of radiation is integrally formed with orattached to a medical device.

Optionally, the at least one sensor module includes at least two sensormodules.

Optionally, the at least two sensor modules includes at least threesensor modules.

Optionally, the at least one of the at least one sensor module furthercomprises a locomotion device capable of imparting translational motionto the sensor module so that the sensor module is moved to a newlocation.

Optionally, the locomotion device is operable by a translational motionsignal from the CPU.

Optionally, the system additionally comprises an imaging module, theimaging module capable of providing an image signal to the CPU, the CPUcapable of translating the image signal to an image of a portion of thebody of the subject.

Optionally, the system further comprises a display device.

Optionally, the display device is capable of displaying the image of theportion of the body of the subject with a determined position of themedical device superimposed on the image of the portion of the body ofthe subject.

Optionally, the CPU receives at least two of the output signals andcomputes a position of the radiation source based on the output signals,

Optionally, the CPU receives at least three of the output signals andcomputes a position of the radiation source based on the at least threeoutput signals.

Optionally, wherein the CPU computes the position repeatedly atintervals so that a position of the radiation source as a function oftime may be plotted.

Optionally, wherein the radiation source employs an isotope with a halflife in the range of 6 to 18 months.

Optionally, the system further comprises additionally comprising theradiation source capable of providing the radiation.

Optionally, the directional information is produced when the source hasan activity in the range of 0.01 mCi to 0.5 mCi.

In an exemplary embodiment of the invention, a sensor for directionallylocating an ionizing radiation source is provided. The sensor comprises:

(a) at least one functional component; and

(b) a displacement mechanism which imparts angular sensitivity to thesensor by moving the at least one functional component.

Optionally, the at least one functional component comprising at leastone radiation detector, the at least one radiation detector capable ofreceiving radiation from the radiation source and producing an outputsignal;

wherein the displacement mechanism is capable of rotating the at leastone radiation detector through a rotation angle so that the outputsignal varies with the rotation angle.

Optionally, the at least one radiation detector comprises at least onefirst radiation detector and at least one second radiation detector andthe output signal comprises at least one first output signal from the atleast one first radiation detector and at least one second output signalfrom the at least one second radiation detector.

Optionally, the sensor comprises at least one radiation shield installedat a fixed angle with respect to the at least one first radiationdetector and the at least one second radiation detector so that amagnitude of the first output signal from the at least one firstradiation detector and a magnitude of the second output signal from thesecond radiation detector vary with the rotation angle.

Optionally, the sensor comprises:

(a) at least one first radiation detector and at least one secondradiation detector, each of the at least one first radiation detectorand at least one second radiation detector capable of receivingradiation from the radiation source and producing at least one firstoutput signal from the at least one first radiation detector and atleast one second output signal from the at least one second radiationdetector;

(b) at least one radiation shield, the radiation shield rotatable aboutan axis of shield rotation through an angle of shield rotation, so thata magnitude of the first output signal from the at least one firstradiation detector and a magnitude of the second output signal from thesecond radiation detector each vary with the angle of shield rotation.

Optionally, the at least one radiation shield comprises:

(i) a primary radiation shield located between the at least one firstradiation detector and the at least one second radiation detector;

(ii) at least one first additional radiation shield deployed tointerfere with incident radiation directed towards the at least onefirst radiation detector; and

(iii) at least one second additional radiation shield deployed tointerfere with incident radiation directed towards the at least onesecond radiation detector.

Optionally, wherein the at least one first additional radiation shieldand the at least one second additional radiation shield are eachinclined towards the primary radiation shield.

Optionally, wherein the at least one first radiation detector and the atleast one second radiation detector are organized in pairs, each pairhaving a first member and a second member and each radiation shield ofthe primary and additional radiation shields is located between one ofthe first member and one of the second member of one of the pairs sothat the output signal varies with the rotation angle.

Optionally, the sensor is additionally capable of revolving the at leasta functional component about an axis of revolution through an angle ofrevolution.

In an exemplary embodiment of the invention, a method of determining alocation of a device is provided. The method comprises:

(a) providing a device having a radiation source associated therewith;

(b) determining a direction towards the radiation source;

(c) further determining at least a second direction towards theradiation source;

(d) locate the device by calculating an intersection of the firstdirection and the at least a second direction.

Optionally, the further determining at least a second direction towardsthe radiation source includes determining at least a third directiontowards the radiation source and additionally comprising:

(e) calculating a point of intersection of the first direction, thesecond direction and the at least a third direction.

In an exemplary embodiment of the invention, a method of manufacturing atrackable medical device is provided. The method comprises incorporatinginto or fixedly attaching a detectable amount of a radioactive isotopeto the medical device.

Optionally, the detectable amount is in the range of 0.01 mCi to 0.5mCi.

Optionally, the detectable amount is 0.1 mCi or less.

Optionally, the detectable amount is 0.05 mCi or less.

Optionally, the isotope is Iridium-192.

An aspect of some embodiments of the invention relates to use of anionizing radiation source with an activity of 0.1 mCi or less as atarget for non imaging localization or tracking.

BRIEF DESCRIPTION OF FIGURES

In the Figures, identical structures, elements or parts that appear inmore than one Figure are generally labeled with the same numeral in allthe Figures in which they appear. Dimensions of components and featuresshown in the Figures are chosen for convenience and clarity ofpresentation and are not necessarily shown to scale. The Figures arelisted below.

FIG. 1 is a side view of one embodiment of a sensor module according toan exemplary embodiment of the present invention;

FIG. 2 is a schematic representation of a computerized tracking systemaccording to an exemplary embodiment of the present invention;

FIG. 3 is a side view of an additional embodiment of a sensor moduleaccording to an exemplary embodiment of the present inventionillustrating receipt of a signal by the module;

FIG. 4 is a perspective view of a computerized tracking system accordingto an exemplary embodiment of the present invention illustrating onepossible arrangement of sensor modules with respect to a patient;

FIG. 5 is a side view of another additional embodiment of a sensormodule according to an exemplary embodiment of the present invention;

FIGS. 6A and 6B are side views of further additional embodiments of asensor module according to exemplary embodiments of the presentinvention;

FIGS. 7A and 7B are graphs of simulated response time and simulated runsposition error respectively plotted as a function of sensor rotation perphoton impact using a system according to an exemplary embodiment of thepresent invention;

FIGS. 8A and 8B are graphs of simulated response time and simulated rmsposition error respectively plotted as a function of sampling time usinga system according to an exemplary embodiment of the present invention;

FIGS. 9A and 9B are graphs of simulated response time and simulated rmsposition error respectively plotted as a function of specific activityof a radioactive signal source using a system according to an exemplaryembodiment of the present invention;

FIG. 10A is a graph of position as a function of time. Simulatedposition output from a system according to an exemplary embodiment ofthe present invention is overlaid on a plot of actual input position forthe simulation;

FIG. 10B is a graph of rms position error plotted as a function of timebased upon the two plots of FIG. 10A;

FIG. 11 is a simplified flow diagram of a method according to exemplaryembodiments of the present invention; and

FIG. 12 is a graph of sensor output as a function of rotation angle.

DETAILED DESCRIPTION OF EMBODIMENTS

According to one embodiment of the invention (FIGS. 2 and 4), acomputerized system 40 locates and/or tracks a device. In the embodimentdepicted in FIG. 4, the device is a medical device. Medical devicesinclude, but are not limited to, tools, implants, navigationalinstruments and ducts. Tools include, but are not limited to, catheters,canulae, trocar, cutting implements, grasping implements and positioningimplements. Implants include, but are not limited to, brachytherapyseeds, stents and sustained release medication packets. Navigationalinstruments include, but are not limited to, guidewires. Ducts include,but are not limited to, tubing (e.g. esophageal tubes and trachealtubes). In exemplary embodiments of the invention, one or more movingtools are tracked.

In an exemplary embodiment of the invention, position of the source isdetermined by non-imaging data acquisition. For purposes of thisspecification and the accompanying claims, the phrase “non-imaging”indicates data not acquired as part of an image acquisition process thatincludes the source and anatomical or other non-source features in asame image. Optionally, a sensor which is not suitable for and notconnected to imaging circuitry is employed. Imaging relies uponinformation about many points, including at least one point of interest,and image analysis of the information determines characteristics of thepoint(s) of interest, for example position relative to an object. In anexemplary embodiment of the invention, position sensing providesinformation only about the source. This can improve detectability and/oraccuracy.

Optionally, the medical device is at least partially within a body of asubject 54 during at least part of the path upon which its location isdetermined. In FIG. 4, an exemplary embodiment in which system 40 isconfigured to track a device through the head of subject 54 during anintracranial medical procedure is depicted. This drawing is purelyillustrative and should not be construed as a limitation of the scope ofthe invention.

FIG. 2 shows an embodiment of system 40 including three sensor modules20 which rely on angular detection acting in concert to determine alocation of radioactive source 38. In the pictured embodiment, each ofsensors 20 determines an angle of rotation 32 indicating a directiontowards source 38. This angle of rotation 32 (FIG. 1) defines a plane inwhich source 38 resides and which crosses radiation detector 22. Angleof rotation 32 is provided as an output signal 34 which is relayed tocomputerized processing unit (CPU) 42. CPU 42 determines an intersectionof the three directions (planes) which is expressed as a point.

According to some embodiments of the invention, a source 38 locatedwithin the boundaries 24 of detection (FIG. 1) of sensor 20 may beaccurately located by system 40 as radiation detector 22 of sensormodule 20 is rotated through a series of rotation angles 32. A source 38located outside of boundaries 24 will not be accurately located. Forthis reason, it is desirable, in some embodiments, that each of sensors20 is deployed so that the predicted path of source 38 lies withinboundaries 24. According to some embodiments of the invention, sensor 20may move to keep source 38 within boundaries 24. The size and shape ofboundaries 24 vary according to the configuration of sensor 20.

Accuracy of determination of target rotation angle 32 contributes toaccuracy of the location of source 38 as determined by system 40.Various modifications to sensor module 20 which can increase thesensitivity to small differences in rotation angle 32 are depicted asexemplary embodiments in FIGS. 3, 5, 6A and 6B and explained in greaterdetail hereinbelow.

FIG. 4 provides a perspective view of an exemplary system 40 whichemploys angular detection and includes three sensor modules 20 dispersedupon the circumference of a circle 58. In the pictured embodiment,modules 20 feature radiation shields 36. In the pictured embodiment,each module 20 rotates about an axis tangent to circle 58. This rotationallows tracking of the medical device as explained in greater detailhereinbelow. According to various embodiments of the invention,rotational motion or translational motion may be employed to facilitatethe desired angular detection. According to the embodiment depicted inFIG. 4, sensor modules 20 are situated below the head of subject 54 suchthat the vertical distance between the plane of sensor modules 20 andthe region of interest within the head is approximately equal to theradius of circle 58. This arrangement assures that each of sensors 20are deployed so that the predicted path of source 38 lies withinboundaries 24. This arrangement may be repeatably and easily achieved byproviding three of sensors 20 mounted on a board equipped with a raisedheadrest in the center of circle 58. This optionally permits a recliningchair or adjustable examination table to be easily positioned so thatsubject 54 is correctly placed relative to sensors 20 without anextensive measuring procedure.

Positioning volume of system 40 is the set of spatial coordinates inwhich a location of source 38 may be determined. Positioning volume ofsystem 40 has a size and/or shape dependent upon positions of sensor(s)20, their design and/or their performance characteristics. Optionally,positioning volume of system 40 can be expressed as the intersection ofboundaries of detection 24 of sensors 20. Optionally, two or morepositioning volumes may be created, by using multiple sets of sensors20. Optionally, these positioning volumes may overlap.

The 3-dimensional position of the center of mass of a radiation source38 is calculated by CPU 42 from the angle 32 measured by each of sensormodules 20, given the known location and rotation axis of each ofmodules 20. According to some embodiments of the invention, source 38will be a piece of wire with a length of 1 to 10 mm. This range oflengths reflects currently available solid isotope sources 38 suppliedas wires with useful diameters and capable of providing a sufficientnumber of DPM to allow efficient operation of system 40. System 40determines the position of the middle of this piece of wire 38 andresolves the determined position to a single point, optionallyindicating margins of error.

Sensor module 20 includes at least one radiation detector 22. Radiationdetector 22 is capable of receiving radiation from radiation source 38attached to the medical device and producing an output signal 34.Radiation detector 22 may employ any technology which transformsincident radiation into a signal which can be relayed to CPU 42. Ifsource 38 is a gamma radiation source, radiation detector 22 may be, forexample, an ionization chamber, a Geiger-Mueller Counter, ascintillation detector, a semiconductor diode detector, a proportioncounter or a micro channel plate based detector. Radiation detectors 22of various types are commercially available from, for example,EVproducts (Saxonburg Pa., USA); Hammatsu Photonics (Hamamatsu City,Shizuoaka, Japan); Constellation Technology, (Largo, Fla., USA); SoltecCorporation (San Fernando Calif., USA); Thermo Electron Corporation,(Waltham Mass., USA): Bruker-biosciences (Billerica Mass., USA); SaintGobain crystals (Newbury Ohio, USA) and Silicon Sensor GMBH (Germany). Asuitable commercially available radiation detector 22 can beincorporated into the context of system 40 as part of sensor 20.Embodiments of the invention which rely upon a source 38 producing asmall number of DPM and S types of detectors 22 which offer goodsensitivity (i.e. high ratio between CPM and DPM) will improve theperformance of sensor modules 20. As the distance between sensor 20 andsource 38 increases, this consideration becomes more relevant.Embodiments of the invention which rely upon source 38 with a greaterDPM output may permit use of less sensitive radiation detectors 22.

Various types of sensor modules 20 are described in greater detailhereinbelow.

System 40 further includes radiation source 38 capable of providing asufficient amount of radiation for location and/or tracking at a ratewhich will not adversely affect a procedure being carried out by themedical device. For most medical procedures, 10 locations/second issufficient to allow an operator of system 40 to comfortably navigate themedical device to a desired location. Based upon results from acomputerized simulation described in greater detail hereinbelow, theamount of radiation to meet these criteria can be made low enough thatit does not pose any significant risk to a patient undergoing aprocedure of several hours duration with source 38 inside their body.Alternatively or additionally, the amount may be made low enough so thatan operator of system 40 is not exposed to any significant risk fromradiation exposure over time as explained hereinbelow.

For example, using Iridium-192 increasing the activity of radiationsource 38 from 0.01 mCi to 0.5 mCi improves accuracy only by a factor of2 (FIG. 9B). However, activity levels below 0.1 mCi adversely affectresponse time (FIG. 9A). Activities greater than 0.1 mCi do notsignificantly improve response time. An activity of 0.05 mCi offers anacceptable trade-off between latency and accuracy as described ingreater detail hereinbelow and provides a good compromise betweenperformance and radiation dose.

A 0.05 mCi source 38 meets permits system 40 to achieve adequate speedand accuracy with an amount of radiation produced so low that it may besafely handled without gloves. Radiation exposure for the patient from a0.05 mCi source 38 is only eight times greater than average absorbedbackground radiation in the United States. For purposes of comparison topreviously available alternatives, a 0.05 mCi source 38 exposes thepatient to an Effective Dose Equivalent (EDE) of 0.0022 mSv/hr. Atypical fluoroscopy guided procedure has an EDE of 1-35 mSV perprocedure and a typical Nuclear Medicine procedure has an EDE of 5 mSv.Thus, some embodiments of the invention may be employed to significantlyreduce patient radiation exposure.

Medical personnel are optionally exposed to even less radiation, withthe level of exposure decreasing in proportion to the square of theintervening distance. For example, a doctor located one meter from a0.05 mCi source 38 and performing procedures for 6 hours per day, 5 daysa week, 52 weeks a year would accumulate a total annual EDE of 0.22 mSv.This is approximately 5% of the radiation exposure level at whichexposure monitoring is generally implemented. This level of exposurecorresponds to 1.4 e⁴ mSV/hr which is orders of magnitude less than the1-12 mSv/hr associated with a typical dose from fluoroscopic procedures.

Iridium-192 has been used as an example because it is already approvedfor use in medical applications and is generally considered safe tointroduce into the body of a subject. However this isotope is only anillustrative example of a suitable source 38, and should not beconstrued as a limitation of system 40. When choosing an isotope for usein the context of system 40, activity (DPM), type of radiation and/orhalf life may be considered. Activity has been discussed above. Inaddition, it is generally desired that disintegration events bedetectable with reasonable efficiency at the relevant distance, forexample 20-50 cm. Long half lives may be preferred because they makeinventory control easier and reduce total costs in the long run byreducing waste. However, short half lives may reduce concerns overradioactive materials and/or may allow smaller sources to be used.

According to some embodiments of the invention, source 38 is a source ofpositron emissions. According to these embodiments, sensors 20 determinea direction from which photons released as a result of positron/electroncollisions originate. This difference optionally does not affectaccuracy of a determined location to any significant degree because thedistance traveled by a positron away from source 38 before it meets anelectron is generally very small. Use of positrons in source 38 caneffectively amplify total ionizing radiation emissions available fordetection. Optionally, the use of multiple detector may allow thedetection of pairs of positron annihilation events to be detected. Otherexamples of source types include gamma sources, alpha sources, electronsources and neutron sources.

Regardless of the isotope, source 38 may be incorporated into a medicaldevice (e.g. guidewire or catheter) which is to be tracked.Incorporation may be, for example, at or near the guidewire tip and/orat a different location in a catheter or in an implant. The source ofionizing radiation may be integrally formed with, or attached to, aportion of the guidewire or to a portion of the medical device.Attachment may be achieved, for example by gluing, welding or insertionof the source into a dedicated receptacle on the device. Attachment mayalso be achieved by supplying the source as an adhesive tag (e.g. acrack and peel sticker), paint or glue applicable to the medical device.Optionally, the source of ionizing radiation is supplied as a solid, forexample a length of wire including a radioactive isotope. A short pieceof wire containing the desired isotope may be affixed to the guidewireor medical device. This results in co-localization of the medical deviceand the source of radiation. Affixation may be accomplished, forexample, by co-extruding the solid source with the guidewire during themanufacture of the guide wire. Alternately, or additionally, the sourceof ionizing radiation may be supplied as a radioactive paint which canbe applied to the medical device and/or the guidewire. Regardless of theexact form in which the ionizing radiation source is supplied, oraffixed to the guidewire or medical device, it should not leave anysignificant radioactive residue in the body of the subject after removalfrom the body at the end of a medical procedure.

While source 38 is illustrated as a single item for clarity, two or moresources 38 may be tracked concurrently by system 40. System 40 mayidentify multiple sources 38 by a variety of means including, but notlimited to, discrete position or path, frequency of radiation, energy ofradiation or type of radiation. According to some embodiments of theinvention, use of two or more resolvable sources 38 provides orientationinformation about the item being tracked. In other words, theseembodiments permit determination of not only a 3-dimensional positiondefined by co-ordinates X, Y and Z, but also information about theorientation of the tracked object at the defined location. This featureis relevant in a medical context when a non-symmetrical tool isemployed.

System 40 may include a channel of communication 48 capable of conveyinga data signal between the one or more sensor modules 20 and acomputerized processing unit (CPU) 42. Channel of communication may bewired or wireless or a combination thereof. Wired channels ofcommunication include, but are not limited to direct cable connection,telephone connection via public switched telephone network (PSTN), fiberoptic connection and construction of system 40 as an integrated physicalunit with no externally apparent wires. Wireless channels ofcommunication include, but are not limited to infrared transmission,radio frequency transmission, cellular telephone transmission andsatellite mediated communication. The exact nature of channel ofcommunication 48 is not central to operation of system 40 so long assignal transmission permits the desired refresh rate. Channels ofcommunication 48 may optionally permit system 40 to be operated in thecontext of telemedicine. Alternately, or additionally, channels ofcommunication 48 may serve to increase the distance between source 38and medical personnel as a means of reducing radiation exposure to themedical personnel to a desired degree.

CPU 42 is designed and configured to receive output signal 34 viachannel of communication 48 and translate output signal 34 todirectional information concerning radiation source 38. This directionalinformation may be expressed as, for example, a plane in which radiationsource 38 resides. Output signal 34 includes at least rotation angle 32.Optionally, output signal 34 may also include a signal strengthindicating component indicating receipt of a signal from source 38.Receipt of a signal from source 38 may be indicated as either a binarysignal (yes/no) or a signal magnitude (e.g. counts per minute).According to various embodiments of the invention, output signal 34 maybe either digital or analog. Translation of an analog signal to adigital signal may be performed either by sensor module 20 or CPU 42. Insome cases, locating radiation source 38 in a single plane issufficient. However, in most embodiments of the invention, it isdesirable that CPU 42 receives two of output signals 34 and computes anintersection. If output signals 34 are expressed as planes, thisproduces a linear intersection 44 of two of the planes. This locatesradiation source 38 upon the linear intersection 44. Optionally, results44 of this calculation are displayed on a display device 43 as describedin greater detail hereinbelow. In additional embodiments of theinvention, CPU 42 receives at least three of output signals 34 andcomputes their intersection. If output signals 34 are expressed asplanes and sensors 20 are positioned on the circumference of circle 58,this produces a point of intersection 44 of at least three planes,thereby locating radiation source 38 at the calculated point ofintersection 44.

Because system 40 is most often employed to track a medical instrumentduring a medical procedure, CPU 42 is often employed to compute thepoint of intersection repeatedly at predetermined intervals so that aposition of radiation source 38 as a function of time may be plotted(see FIG. 10 a). The accuracy of each plotted position and of the plotas a whole may be influenced by the activity of source 38, the accuracyand response time of sensors 20 and the speed at which the implantedmedical device is moving through subject 54. Because medical proceduresgenerally favor precision over speed, an operator of system 40 maycompensate for deficiencies in source 38, or accuracy or response timeof sensors 20, by reducing the rate of travel of the medical devicebeing employed for the procedure. FIG. 10B illustrates output of asimulated system 40 with tracking accuracy in the range of +2 mm. CPU 42may also optionally employ channel of communication 48 to send varioussignals to sensor module(s) 20 as detailed hereinbelow. Alternately, oradditionally, CPU 42 may also optionally employ channel of communication48 to send various signals to the medical device. According to variousembodiments of the invention, system 40 may be employed in the contextof procedures including, but not limited to, angioplasty (e.g. balloonangioplasty), deployment procedures (e.g. stent placement orimplantation of radioactive seeds for brachytherapy), biopsy procedures,excision procedures and ablation procedures.

While CPU 42 is depicted as a single physical unit, a greater number ofphysically distinct CPUs might actually be employed in some embodimentsof the invention. For example, some functions, or portions of functions,ascribed to CPU 42 might be performed by processors installed in sensormodules 20. For purposes of this specification and the accompanyingclaims, a plurality of processors acting in concert to locate source 38as described herein should be viewed collectively as CPU 42.

According to some embodiments of the invention, system 40 concurrentlyemploys three or more sensor modules 20 in order to concurrently receivethree or more output signals 34 and compute three or more directionsindicating signal source 38. If the directions are expressed as planes,the three or more planes intersect in a single point. However, system 40includes alternate embodiments which employ two, or even one, sensormodule 20 to localize source 38 to a single point. This may be achievedin several different ways as described hereinbelow.

According to some embodiments of system 40 at least one of sensor module20 is capable of rotating the at least one radiation detector 22 througha series of positions. Each position is defined by a rotation angle 32so that receiving the radiation from source 38 upon detector 22 varieswith rotation angle 32. This rotation may be accomplished in a varietyof ways. For example, rotation mechanism 26 may be operated by feedbackfrom 28 from radiation detector 22 according to a rule with amount ofreceived radiation as a variable. Alternately, rotation mechanism 26 maybe operated by a signal from CPU 42 according to a rule including amountof received radiation and/or time as variables. Alternately, rotationmechanism 26 may rotate radiation detector 22 according to a fixedschedule, with no regard to how much radiation impinges upon radiationdetector 22 at any particular rotation angle 32. Rotation mechanism 26may employ a wide variety of different mechanisms for achieving rotationangle 32. These mechanisms include, but are not limited to, mechanicalmechanisms, hydraulic mechanisms, pneumatic mechanisms, electricmechanisms, electronic mechanisms and piezoelectric mechanisms.Optionally, an independent angle measuring element 30 may be employed tomore accurately ascertain the actual rotation angle 32. Although anglemeasuring element 30 is depicted as a physically distinct component inFIGS. 1, 2 and 3, it could be physically integrated into rotationmechanism 26 without affecting performance of system 40 to anysignificant degree. Regardless of the exact operational details, theobjective is to detect the rotation angle 32 at which sensor module 20is pointing directly towards source 38. This angle will be referred toas the target rotation angle 32.

According to some embodiments of system 40, radiation detector 22 (FIGS.3, 5, 6A and 6B) includes at least one first radiation detector 22A andat least one second radiation detector 22B. These embodiments of system40 rely upon comparison of output signals 34 from radiation detectors22A and 22B for each rotation angle 32. A target angle of rotation 32which produces output signals 34 from radiation detectors 22A and 22Bwith a known relationship indicates that radiation detectors 22A and 22Bare both facing source 38 to the same degree. When radiation detectors22A and 22B have identical receiving areas, the known relationship isequality. This target angle of rotation 32 is employed to determine aplane in which source 38 resides.

In order to increase the sensitivity of system 40 to small differencesbetween output signals 34 from radiation detectors 22A and 22B it ispossible to introduce one or more radiation shields 36 at a fixed anglewith respect to radiation detectors 22A and 22B. Radiation Shield 36causes a magnitude of the component of output signal 34 from firstradiation detector 22A and a magnitude of the component of output signal34 from second radiation detector 22B to each vary with rotation angle32 (see FIG. 3). Radiation shield 36 differentially shadows eitherradiation detectors 22A or 22B depending upon the relationship betweenangles of incidence 39 and 41. At some angle of rotation 32, neitherradiation detector 22A nor 22B will be shadowed by radiation shield 36.This angle of rotation 32 is employed to determine a plane in whichsource 38 resides. This configuration insures that small variations fromthis target angle of rotation 32 cause relatively large differences inthe output signals 34 from radiation detectors 22A and 22B because ofthe shadow effect. Therefore, use of radiation shield 36 in sensormodule 20 increases the sensitivity of system 40. This increasedsensitivity permits sensor module 20 to function effectively even with alow number of detectable radioactive counts.

FIG. 6A illustrates an additional embodiment of sensor module 20 inwhich the radiation shield includes a primary radiation shield 36located between first radiation detector 22A and second radiationdetector 22B. The picture embodiment also includes a series of firstadditional radiation shields (36A1, 36A2, and 36A3) which divide firstradiation detector 22A into a series of first radiation detectors andinterfere with incident radiation directed towards first radiationdetector 22A. The pictured embodiment also includes a series of secondadditional radiation shields (36B1, 36B2, and 36B3) which divide secondradiation detector 22B into a series of second radiation detectors andinterfere with incident radiation directed towards second radiationdetector 22B. This configuration can insure that even smaller variationsfrom target rotation angle 32 cause relatively large differences in theoutput signals 34 from radiation detectors 22A and 22B by increasing theshadow effect in proportion to the number of additional radiationshields (36A1, 36A2, 36A3, 36B1, 36B2, and 36B3 in the picturedembodiment). Therefore, use of additional radiation shields (e.g. 36A1,36A2, 36A3, 36B1, 36B2, and 36B3) in sensor module 20 may serve toachieve an additional increase in sensitivity of system 40. Optionally,secondary radiation shields (36A1, 36A2, 36A3, 36B1, 36B2, and 36B3 inthe pictured embodiment) are inclined towards primary radiation shield36. The angle of secondary radiation shields 36A1, 36A2, 36A3, 36B1,36B2, and 36B3 towards primary shield 36 can be changed, for example,using a motor to improve focus and/or define imaging volume.

A similar effect may be achieved by holding radiation detectors 22A and22B at a fixed angle and subjecting radiation shield(s) 36 (FIG. 6B) toangular displacement. Therefore, system 40 also includes embodiments inwhich radiation detector 22 includes at least one first radiationdetector 22A and at least one second radiation detector 22B and outputsignal 34 includes discrete components from detectors 22A and 22B withat least one radiation shield 36 rotatable about an axis of shieldrotation through an angle of shield rotation 32 so that a magnitude ofdiscrete components of output signal 34 from detectors 22A and 22B eachvary as a function of the angle of shield rotation 32.

Referring now to FIG. 5, alternate embodiments of sensor module 20 ofsystem 40 are configured so that radiation detector 22 includes aplurality of radiation detectors 22 and a plurality of protrudingradiation shields 36 interspersed between the plurality of radiationdetectors 22. According to these embodiments, plurality of radiationdetectors 22 is organized in pairs, each pair having a first member 21and a second member 23 and each protruding radiation shield 36 of theplurality of protruding radiation shields is located between firstmember 21 and second member 23 of the pair of radiation detectors 22.According to this embodiment, sensor module 20 is capable of rotatingthe radiation detectors 22 through a series of rotation angles 32 sothat the receiving the radiation from radiation source 38 upon radiationdetectors 22 varies with rotation angle 32. Each radiation detectorproduces an output signal 34. CPU 42 sums output signals 34 from allfirst members 21 to produce a first sum and all second members 23 toproduce a second sum. Assuming that all of radiation detectors 22 areidentical, when the sensor is aimed directly at the center of mass ofsource 38 (target rotation angle 32), the first sum and the second sumare equivalent. This embodiment insures that the total output for theentire module 20 increases rapidly with even a very slight change inrotation angle 32 in either direction. Alternately, or additionally, thesign of the total output for the entire module 20 indicates thedirection of rotation required to reach the desired rotation angle 32 atwhich total output for the entire module 20. Thus, this configurationserves to increase both speed of operation and overall accuracy ofsystem 40. This type of sensor module 20 may be operated (for example)by implementation of a first algorithm summing gamma ray impacts fromsource 38 for a period of time and allowing CPU 42 to decide, based onthe sign of total output for the entire module 20, in which directionand to what degree to rotate radiation detectors 22 in an effort toreach a desired rotation angle 32. Alternately, CPU 42 may (for example)implement a second algorithm rotates radiation detectors 22 a very smallamount in response to every detected count. Performance data presentedherein is based upon a simulation of the second algorithm, but the firstalgorithm is believed to be equally useful.

According to additional embodiments of system 40, a single sensor module20 may be employed to determine two intersecting planes in which source38 resides. This may be achieved, for example, by revolution of sensormodule 20 or by moving sensor module 20 to a new location.

According to some embodiments of the invention, sensor module 20 may beadditionally capable of revolving radiation detector 22 about an axis ofrevolution 25 through an angle of revolution 29. Revolution is producedby a revolution mechanism 27 which may function in a variety of ways asdescribed hereinabove for rotation mechanism 26. According to theseembodiments of the invention angle of revolution 29 is included as acomponent of the orientation of sensor module 20 and is included inoutput signal 34. Revolution may be employed in the context of any orall of the sensor module 20 configurations described hereinabove andhereinbelow. Revolution may occur, for example, in response to arevolution signal 46 transmitted to sensor module 20 from CPU 42 viachannel of communication 48.

According to additional embodiments of the invention, sensor module 20includes a locomotion device 31 capable of imparting translationalmotion 33 to module 20 so that the location of module 20 is changed.Locomotion may be initiated, for example, in response to a translationalmotion signal 46 transmitted to sensor module 20 from CPU 42 via channelof communication 48. According to various embodiments of the invention,locomotion may be used to either permit a single sensor module 20 tooperate from multiple locations or to provide angular sensitivity tosensor module 20. In other words, translational motion may be used as asubstitute for angular displacement, especially in embodiments whichemploy at least one radiation shied 36. In embodiments which employstranslational motional, translation of a single sensor 20 in a firstdimension permits acquisition of a first set of directional information.For example, in the embodiment of system 40 depicted in FIG. 4,successive vertical displacement of sensor 20A could be used todetermine a first plane in which source 38 resides. Successivehorizontal displacement of sensor 20B could be used to determine asecond plane in which source 38 resides. Alternately, or additionally, asingle sensor 20 may be subject to both vertical and horizontaldisplacement. Successive vertical and horizontal displacement permits asingle sensor 20 to determine two non-parallel planes in which source 38resides. Concurrent vertical and horizontal displacement along a singleline permits a single sensor 20 to determine a single plane in whichsource 38 resides. Determination of intersection of 2 or 3 or moreplanes is as determined above. Optionally locomotion and revolution maybe employed in the same embodiment of the invention.

Optionally, system 40 further includes an imaging module 50 including animage capture device 56 capable of providing an image signal 52 to CPU42. Imaging module 50 optionally includes an interface to facilitatecommunication with CPU 42. CPU 42 is capable of translating image signal52 to an image of a portion of the body of subject 54. According tovarious embodiments of the invention, imaging module 50 may rely uponfluoroscopy, MRI, CT or 2D or multi-plane or 3D angiography. Forintracranial procedures, imaging generally need not be conductedconcurrently with the procedure. This is because the brain does notshift much within the skull. Images captured a day or more before aprocedure, or a few hours before a procedure, or just prior to aprocedure, may be employed. According to alternate embodiments of theinvention, image data is acquired separately (i.e. outside of system 40)and provided to CPU 42 for alignment.

Alignment methods and the algorithms for anatomical image display andtracking information overlay are reviewed in Jolesz (1997) Radiology.204(3):601-12. The Jolesz article, together with references citedtherein, provides enablement for a skilled artisan to accomplishconcurrent display and alignment of image data and tracking data. TheJolesz reference, together with references cited therein, are fullyincorporated herein by reference to the same extent as if eachindividual reference had been individually cited and incorporated byreference.

In an exemplary embodiment of the invention, the location(s) determinedby system 40 are registered with respect to the image. This may beaccomplished, for example by registering system 40 and/or sensors 20 toimage capture device 56.

Regardless of which type of sensor module 20 is employed, system 40 mayinclude a display device 43 in communication with CPU 42. Display device43 may display the image of the portion of the body of the subject witha determined position of the medical device (corresponding to a positionof source 38) superimposed on the image of the portion of the body ofthe subject. The superimposed determined position is optionallyrepresented as a point on display screen 43. Optionally the point issurrounded by an indicator of a desired confidence interval determinedby CPU 42. The confidence interval may be displayed, for example, as acircle, as two or more intersecting lines or as one or more pairs ofbrackets. Alternately, or additionally, display device 43 may displayposition coordinates of a determined position of the medical device(e.g., corresponding to a position of source 38 at a tip of guidewire).

Display device 43 may be provided with a 3-dimensional angiographydataset from CT, MRI, or 3-D angiography, imaged either during theprocedure or prior to the procedure. Appropriate software can beemployed to extract a 3-D model of the vasculature from the angiographydataset, and display this model using standard modes of 3-D modelvisualization. A 3-dimensional graphical representation of the guidewireor catheter can be integrated into the 3-D model of the vasculature andupdated with minimal temporal delay based on the position informationprovided by system 40 to indicate the position of the guidewire orcatheter within the vasculature. The entire 3-D model including thevasculature and the catheter can be zoomed, rotated, and otherwiseinteractively manipulated by the user during performance of theprocedure in order to provide the best possible visualization.

Optionally, system 40 may further include one or more user input devices45 (e.g. keyboard, mouse, touch screen, track pad, trackball,microphone, joystick or stylus). Input device 45 may be used to adjustan image as described hereinabove on display device 43 and/or to issuecommand signals to various components of system 40 such as rotationmechanism 26, revolution mechanism 27, locomotion device 31 or imagecapture device 56.

The invention optionally includes a sensor 20 for determining a plane inwhich a radiation source resides as depicted in FIG. 3 and describedhereinabove. Briefly, the sensor 20 includes at least one radiationdetector 22, the at least one radiation detector capable of receivingradiation from radiation source 38 and producing an output signal 34.Sensor 20 is capable of rotating radiation detector 22 through a seriesof positions, each position defined by a rotation angle 32 so that thereceiving the radiation from radiation source 38 upon radiation detector22 varies with rotation angle 32. Rotation is optionally achieved asdescribed hereinabove. A rotation angle 32 which produces a maximumoutput signal indicates the plane in which radiation source 38 resides.

According to some embodiments of sensor 20, radiation detector 22includes at least one first radiation detector 22A and at least onesecond radiation detector 22B and output signal 34 includes a firstoutput signal from first radiation detector 22A and a second outputsignal from radiation detector 22B.

According to some embodiments of sensor 20, at least one radiationshield 36 is further installed at a fixed angle with respect todetectors 22A and 22B. As a result, a magnitude of the first outputsignal 34 from the at least one first radiation detector and a magnitudeof the second output signal 34 from radiation detector 22B each varywith rotation angle 32 as detailed hereinabove.

A sensor 20 for determining a plane in which a radiation source residesand characterized by at least one radiation shield 36 rotatable about anaxis of shield rotation through an angle of shield rotation 32 asdescribed hereinabove in detail (FIG. 6B) is an additional embodiment ofthe invention

Sensor 20 for determining a plane in which a radiation source resides asdepicted in FIG. 5 and described hereinabove is an additional embodimentof the invention.

According to alternate embodiments of the invention, a method 400 (FIG.11) of determining a location of a medical device within a body of asubject is provided. Method 400 includes co-localizing 401 a radioactivesignal source 38 with a medical device. Co-localization may be achieved,for example, by providing a device having a radiation source associatedtherewith or by associating a radiation source with a device.

Method 400 further includes determining 402 a first plane in which theomni directional signal generator resides, further determining 403 asecond plane in which the omni directional signal generator resides,calculating 404 a linear intersection of the first plane and the secondplane as a means of determining a line upon which the medical deviceresides.

Method 400 optionally includes further determining 405 at least oneadditional plane in source 38 resides.

Method 400 optionally includes calculating 406 a point of intersectionof the first plane, the second plane and the at least one additionalplane as a means of determining a location of the medical device.

Optionally, method 400 is successively iterated 408 so that a series oflocation are generated to track an implanted medical device in motion.Calculated locations may be displayed 410 in conjunction with anatomicalimaging data if desired.

The various aspects and features of system 40 and/or sensors 20described in detail hereinabove may be employed to enable or enhanceperformance of method 400.

System 40 and method 400 may employ various mathematical algorithms tocompute the location of source 38. One example of an algorithm suitedfor use in the context of some embodiments of the invention calculatesthe position of source 38 from sensor output signal 34, sensor position,and sensor orientation (i.e. rotation angle 32) of three sensors asfollows:

-   -   1) the plane defined by each sensor module 20 is calculated        using an equation of the form

Ax+By+Cz=D

-   -   2) the coefficients A, B, C, and D are calculated as follows:        -   a. Three non-collinear points are defined within sensor 20's            internal reference frame.    -   b. These three points are then shifted by the position of sensor        20 and rotated by the sensor orientation. This defines the plane        in which source 38 would lie if output signal 34 was zero.        -   c. These three points are then rotated about the axis of            rotation angle 32 of sensor 20 by rotation angle 32            indicated by output signal 34. This defines the plane in            which source 38 lies as measured by a particular sensor 20.        -   d. Using the x,y,z coordinates of the three points, x1, y1,            z1, x2, y2, z2, x3, y3, z3 in the following equations, A, B,            C, and D are calculated as follows:            -   i. A=y1(z2−z3)+y2(z3−z1)+y3(z1−z2)            -   ii. B=z1(x2−x3)+z2(x3−x1)+z3(x1−x2)            -   iii. C=x1(y2−y3)+x2(y3−y1)+x3(y1−y2)            -   iv. D=x1(y2*z3−y3*z2)+x2(y3*z1−y1*z3)+x3(y1*z2−y2*z1)    -   3) Calculation of A, B, C, and D for each of three sensors 20        produces a system of three equations in three unknowns:

${\begin{bmatrix}{A\; 1} & {B\; 1} & {C\; 1} \\{A\; 2} & {B\; 2} & {C\; 2} \\{A\; 3} & {B\; 3} & {C\; 3}\end{bmatrix}\begin{bmatrix}x \\y \\z\end{bmatrix}} = \begin{bmatrix}{D\; 1} \\{D\; 2} \\{D\; 3}\end{bmatrix}$

-   -   -   This system of equations can be solved to provide an exact            solution for (x,y,z) (or part of the vector), the point of            intersection of the three planes, which is the position of            the source 38.

Use of additional sensors 20 improves the accuracy by averaging theerrors in the individual sensors, and may also provide a means ofestimating the accuracy of the position measurement by indicating theextent to which the sensors agree with each other.

When 4 or more sensors are used, the algorithm is as follows:

Steps 1 and 2 above remain the same—the equation of the plane indicatedby each sensor is calculated. Step 3 is modified as follows:

-   -   3) Once A, B, C, and D have been calculated for each of the        sensors an over-determined system of more than three equations        in three unknowns results:

${\begin{bmatrix}{A\; 1} & {B\; 1} & {C\; 1} \\{A\; 2} & {B\; 2} & {C\; 2} \\{A\; 3} & {B\; 3} & {C\; 3} \\\vdots & \vdots & \vdots\end{bmatrix}\begin{bmatrix}x \\y \\z\end{bmatrix}} = \begin{bmatrix}{D\; 1} \\{D\; 2} \\{D\; 3} \\\vdots\end{bmatrix}$

-   -   -   This over-determined system can be solved in a least square            sense using methods familiar to those skilled in the art in            order to obtain the best solution for (x,y,z), which is the            most likely position of the tracked element. There is            generally no exact solution due to the error in the sensor            outputs, there may be no single point through which all of            the planes pass.        -   In order that the least square solution may be based on the            error defined by the Euclidian distance between each plane            and the solution for (x,y,z), it is necessary to scale all            of the coefficients defining each plane by the lengths of            their respective Normal vectors (the Normal vector is the            vector defined by (A,B,C)). This is done by dividing A, B,            C, and D by sqrt (Â2+B̂2+Ĉ2) before performing the least            square solution.

    -   4) The Euclidian distance between each of the planes and the        calculated position can be used as a measure of the accuracy of        the position measurement. Once the coefficients have been scaled        by the length of the Normal vector, this distance can be        calculated for each sensor as Ax+By+Cz−D. The mean value of the        distances from each plane to the calculated position gives a        measure of the extent to which all of the sensors agree on the        position that was calculated.

Overdetermined systems of equations may be solved using least squaresolution algorithms. Suitable least square algorithms are available ascomponents of commercially available mathematics software packages.

Optionally, other methods of solving equation sets as known in the artare used. Optionally, instead of a set of equations, other calculationmethods are used, for example, neural networks, rule based methods andtable look up methods in which the signals from the sensors are used tolook-up or estimate a resulting position. In systems where the sensorsmove linearly, other solution methods may be used, for example,translating linear positions of the sensors into spatial coordinates ofthe source.

In order to increase the accuracy and performance of system 40 andmethod 400, advance calibration may optionally be performed. Theposition and orientation of each of the sensor modules 20 can becalibrated instead of relying upon values based on the mechanicalmanufacturing of the system. The calibration procedure involves usingsystem 40 to measure the 3-dimensional position of a source 38 at anumber of known positions defined to a high degree of accuracy. Sincethe position of source 38 is known, the equations normally used tocalculate the positions (described above) can now be used with thesensor positions and orientations as unknowns in order to solve forthese values. Various minimization procedures are known in the art. Thenumber of measurements needed to perform such a calibration may dependson the number of sensor modules 20 in system 40, since it is useful tomake enough measurements to provide more equations than unknowns. Thiscalibration procedure also defines the origin and frame of referencerelative to which system 40 measures the position of the source, and cantherefore provide alignment between the tracking system and anothersystem to which it is permanently attached, such as a fluoroscopy systemor other imaging system.

In an exemplary embodiment of the invention, once a position of thesource is known, the sensors can remain aimed at the source and notchange their orientation. Optionally, if the source moves, determinedfor example, by a significant change in detected radiation (e.g., a dropof 30%, 50%, 70%, 90% or a greater or intermediate drop), the sensor ismoved to scan a range of angles where the source is expected to be in.Optionally, the sensor generates a signal indicating on which side ofthe sensor the source is located, for example as described below.Optionally, the range of scanning depends on an expected angularvelocity of the source, for example, based on the procedure, based onthe history and/or based on a user threshold. If scanning within therange fails, the range is optionally increased.

Optionally, if multiple target sources are provided (e.g., ones withdifferent count rates and/or different energy of emission), the sensorsjump between target angles. Optionally, a steady sweep between a rangeof angles encompassing the two (or more) sources is provided.Optionally, sweeping is provided by ultrasonic or sonic vibrations ofthe sensor or part thereof, for example, comprising a range of angles 1,5, 10, 20, 50 or more times a second. Optionally, the amplitude of thevibration determines the range of angles. Optionally, the sensors orsensor portion is in resonance with one or more vibration frequencies.

Optionally, scanning of the sensors, at least in a small range ofangles, such as less than 10 or less than 5 or less than 1 degree, isprovided even when the sensor is locked on a target source.

The tracking accuracy of system 40 using Iridium-192 as source 38 asdescribed hereinabove has been evaluated only by computer simulation.The simulation is a model of the random distribution of gamma photonsemitted by a source 38 within a model head and absorbed by thephoton-sensitive elements 22 in a compound differential sensor unit 20of the type illustrated in FIG. 5. According to the simulation,radiation detector 22 of sensor module 20 rotates so that a new rotationangle 32 is defined every time a photon is absorbed by detector 22. Ifthe photon is absorbed by a positive radiation detector 21 thenradiation detector 22 of sensor module 20 rotates in the positivedirection, and if it is absorbed by a negative sensor 23 then radiationdetector 22 of sensor module 20 rotates in the negative direction. Totaloutput signal 34 of sensor module 20 is its average orientation duringthe sample time.

According to the simulation, performance is defined by two parameters,however other parameters may be used in a practical system:

-   -   1) The Root Mean Square (RMS) error when the target is        stationary    -   2) The time to indicate a 9 mm change in calculated location        after a 10 mm change in actual location of source 38.

The following parameter values are fixed in the simulation:

-   -   1) Distance from the source to the sensor=25 cm (worst case        distance)    -   2) Source distance for which sensor is geometrically        optimized=25 cm    -   3) Width of photon-sensitive surface in each sub sensor=2 mm (18        in FIG. 5)    -   4) Sensor length=10 cm (14 in FIG. 5)    -   5) Height of dividing walls between sensors=5 cm (35 in FIG. 5)    -   6) Width of dividing walls at their base=4 mm (37 in FIG. 5)    -   7) Number of subsensors defined by walls in the compound        sensor=7 (36 in FIG. 5)    -   8) Sensor sensitivity (fraction of incoming gamma rays which are        detected)=0.3

The simulation evaluated and optimized the following parameters withrespect to influence on performance:

-   -   1) Rotation magnitude per absorbed photon (FIGS. 7 a and 7 b)    -   2) Sample time (FIGS. 8A and 8B)    -   3) Photons per second (source activity level) (FIGS. 9A and 9B)    -   4) Overall tracking accuracy (FIGS. 10 a and 10 b)

The simulation determined that as rotation per photon impact increases,response time is improved (FIG. 7 a). However, as rotation per photonimpact increases, RMS position error also increases (FIG. 7 b). There isclearly a trade-off between latency and accuracy. This parameter can bemodified in real-time in order to optimize the trade-off using a motiondetection algorithm as described hereinbelow.

The simulation determined that sample time no significant impact onlatency or accuracy (FIGS. 8A and 8B). This is because for small valuesof rotation per impact, the number of impacts per sample has minimaleffect on accuracy and only determines the latency (the total amount ofrotation per sample). However, if the number of impacts per sample isreduced as a result of a reduction in the sample time, then thereduction in sample time exactly compensates for the reduced responseper sample leaving the latency unchanged.

Radioactivity (number of photons emitted per second) has a very slighteffect on accuracy, improving accuracy only by a factor of 2 as theactivity increases from 0.01 mCi up to 0.5 mCi (FIG. 9B). It has adrastic effect on response time at low activity levels (FIG. 9A) wherethere simply are not enough photons to induce rapid rotation, however atactivity levels above 0.1 mCi there is minimal improvement withincreased activity level. Optimization of this trade-off between latencyand accuracy (see below) is achieved with 0.05 mCi. This specificactivity provides a good compromise between performance and radiationdose, providing a performance suitable for a typical medical applicationwithout imposing a safety risk to the patient or doctor.

In order to optimize the tradeoff between accuracy and latency a motiondetection algorithm was employed to increase the rotation per photonduring motion of tracked source 38. This decreased latency time andincreased accuracy. In the simulation, the percentage of photons hittingreceiving elements 22 classed as positive 21 versus those classed asnegative 23 was used as an indication of motion of tracked source 38. Asthe percentage moved farther away from 50% the rotation per photon wasincreased, reducing latency at the expense of accuracy during motion. Inother words, system 40 begins by moving towards an estimated targetrotation angle 32 in large steps. As estimated target rotation angle 32is approached, the size of the steps is decreased. If target rotationangle 32 is passed, a small compensatory step in the opposite directionis employed. Results are summarized graphically in FIGS. 10 a and 10 b.Briefly, the RMS error of system 40 tracking a moving source 38 is 0.71mm on average. Location of a stationary source 38 by system 40 producesan rms error of 0.62 mm.

In summary, the simulation results indicate that with an activity of0.05 mCi of 192Ir, compound differential sensors of the type illustratedin FIG. 5, and a motion detection algorithm which trades-off latencyagainst accuracy, system 40 can achieve overall accuracy ofapproximately 1 mm RMS.

Simulated sensitivity of sensor module 20 to changes in rotation angle32 is illustrated in FIG. 12 which is a plot of output signal 34 as afunction of rotation relative to target rotation angle 32 for a sensorof the type indicated in FIG. 5. The graph was produced using theformula:

Total Output 34=A/(A+B)

Where A is the sum of all right side sensors 21; and

B is the sum of all left side sensors 23 and B

The total range of output 34 (Y axis) from sensor 20 was arbitrarilydefined as being in a range from 0 to 1. On the X axis, 0 indicates theangle of rotation 32 which indicates the direction of source 38. Thetotal rotational range of sensor 20 was ±32 milliradians from thistarget rotation angle 32. Deviation of more than 32 milliradians awayfrom target rotation angle 32 produced an output 34 of either 0 or 1,indicating the direction of rotation for a return to target rotationangle 32, but not the amount of rotation to reach target rotation angle32. When output 34 is 0 or 1, the only conclusion that can be drawnabout deviation from target rotation angle 32 is that it is greater than32 milliradians in the indicated direction.

The graph of FIG. 12 depicts output 34 for target rotation angle 32 asthe middle of the dynamic range (0.5). Therefore, if output 34 is 0.6, acorrectional rotation of 10 milliradians in the plus direction isindicated to achieve target rotation angle 32. An output 34 of 0.6indicates a correctional rotation with the same magnitude (10milliradians), but in the minus direction. Another way of depicting thesame information would be to indicate a total dynamic range of +0.5 to−0.5 on the Y axis. This middle of the range could be zero, with onedirection being positive and the other negative, or it can be anyarbitrary number, with one direction being higher and the other lower.

As illustrated in FIG. 12, at target angle 32 simulated sensitivity ofsensor 20 to rotation is approximately 1% of the dynamic range permilliradian of rotation.

This 1% sensitivity per milliradian is sufficient to provide the desiredaccuracy (1 mm rms), using a 5 cm×10 cm sensor module 20 with shields 36having a height 35 of 5 cm interspersed between radiation detectors 22and located 25 cm from source 38 with an activity of 0.05 mCi. Adjustingaccuracy parameters, increasing the size of detectors 22, reducing thedistance between sensor 20 and source 38 and increasing the activity ofsource 38 could each serve to reduce the level of directionalsensitivity desired of sensor 20.

Simulation results (not shown) using a sensor 20 of the type shown inFIG. 6A were similar to those described hereinabove.

System 40 and/or sensors 20 rely upon execution of various commands andanalysis and translation of various data inputs. Any of these commands,analyses or translations may be accomplished by software, hardware orfirmware according to various alternative embodiments. In an exemplaryembodiment of the invention, machine readable media contain instructionsfor transforming output signal 34 from one or more sensor modules 20into position co-ordinates of source 38, optionally according to method400. In an exemplary embodiment of the invention, CPU 42 executesinstructions for transforming output signal 34 from one or more sensormodules 20 into position co-ordinates of source 38, optionally accordingto method 400.

According to an exemplary embodiment of the invention a trackablemedical device is manufactured by incorporating into or fixedlyattaching a detectable amount of a radioactive isotope to the medicaldevice. The radioactive isotope may or may not have a medical functionaccording to various embodiments. Optionally, the radioactivity of theisotope has no medical function. Optionally, the radioactive isotope maybe selected so that it can be used in the body without a protectivecoating without adverse reaction with tissue. In an exemplary embodimentof the invention, the detectable amount of isotope is in the range of0.5 mCi to 0.001 mCi. Use of isotope source 38 with an activity in thelower portion of this range may depend on lower speeds of the device,sensitivity of detector(s) 22, distance from sensor 20. Optionally, atleast 1, optionally at least 5, optionally at least 10, optionally atleast 100 detectable counts per second are produced by the incorporatedradioactive isotope.

In the description and claims of the present application, each of theverbs “comprise”, “include” and “have” as well as any conjugatesthereof, are used to indicate that the object or objects of the verb arenot necessarily a complete listing of members, components, elements orparts of the subject or subjects of the verb.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to necessarily limit the scope of the invention. The describedembodiments comprise different features, not all of which are requiredin all embodiments of the invention. Some embodiments of the inventionutilize only some of the features or possible combinations of thefeatures. Variations of embodiments of the present invention that aredescribed and embodiments of the present invention comprising differentcombinations of features noted in the described embodiments can becombined in all possible combinations including, but not limited to useof features described in the context of one embodiment in the context ofany other embodiment. The scope of the invention is limited only by thefollowing claims.

1. A computerized system for determining location by tracking a sourceof ionizing radiation, the system comprising: at least one firstradiation detector capable of receiving ionizing radiation from theradiation source and for producing first output signals; at least onesecond radiation detector capable of receiving ionizing radiation fromthe radiation source and producing second output signals; and at leastone processor configured to receive and use said first and second outputsignals to determine a plane in which the source resides.
 2. The systemof claim 1, wherein the at least one first radiation detector and the atleast one second radiation detector are oriented to enable detection ofthe source when the source is connected to a medical device.
 3. Thesystem of claim 1, wherein each of the at least one first detector andthe at least one second detector are part of at least one sensor module.4. The system of claim 3, wherein said at least one sensor moduleincludes at least three sensor modules.
 5. The system of claim 3,further comprising at least one locomotion device, said locomotiondevice configured to impart translational motion to said at least onesensor module in response to an instruction generated by the at leastone processor.
 6. The system of claim 4, further comprising at leastthree locomotion devices each associated with one of the at least threesensor modules, said at least three locomotion devices configured toimpart translational motion to an associated sensor module in responseto instructions from the at least one processor.
 7. The system of claim1, additionally comprising: an imaging module, said imaging module beingconfigured to provide an image signal to said at least one processorwherein said at least one processor is configured to translate the imagesignal into an image of a portion of a body of a subject.
 8. (canceled)9. The system of claim 7, further comprising a display device configuredto display the body portion image.
 10. The system of claim 9, whereinthe source of radiation is connected to a medical device, and whereinsaid display device is configured to display the image of said portionof the body with a determined position of the medical devicesuperimposed on said image of said portion of the body.
 11. The systemof claim 1, wherein said at least one processor receives at least two ofsaid output signals each defining a plane in which said radiation sourceresides, and computes a linear intersection of the two planes upon whichsaid radiation source is located.
 12. The system of claim 3, whereinsaid at least one sensor module includes three sensor modules, andwherein the at least one processor is configured to receive at leastthree of said output signals each defining a plane in which saidradiation source resides, and to compute a position of said radiationsource by determining an intersection of three planes.
 13. The system ofclaim 12, wherein said at least one processor is configured to computesaid position repeatedly at predetermined intervals so that a positionof said radiation source as a function of time may be plotted.
 14. Thesystem of claim 1, wherein said radiation source employs an isotope witha half life in the range of 6 to 18 months.
 15. The system of claim 1,additionally comprising said radiation source capable of providing saidradiation.
 16. The system of claim 1, wherein the at least one processoris further configured to determine the plane when the source has anactivity in the range of 0.01 mCi to 0.5 mCi.
 17. A sensor fordirectionally locating an ionizing radiation source, the sensorcomprising: (a) at least one functional component; and (b) adisplacement mechanism which imparts angular sensitivity to the sensorby moving said at least one functional component.
 18. A sensor accordingto claim 17, wherein said at least one functional component comprisingat least one radiation detector, said at least one radiation detectorcapable of receiving radiation from the radiation source and producingan output signal; wherein said displacement mechanism is capable ofrotating said at least one radiation detector through a rotation angleso that said output signal varies with said rotation angle.
 19. Thesensor of claim 18, wherein said at least one radiation detectorcomprises at least one first radiation detector and at least one secondradiation detector and said output signal comprises at least one firstoutput signal from said at least one first radiation detector and atleast one second output signal from said at least one second radiationdetector.
 20. The sensor of claim 19, additionally comprising at leastone radiation shield installed at a fixed angle with respect to said atleast one first radiation detector and said at least one secondradiation detector so that a magnitude of said first output signal fromsaid at least one first radiation detector and a magnitude of saidsecond output signal from said second radiation detector vary with saidrotation angle.
 21. A sensor according to claim 17, comprising: (a) atleast one first radiation detector and at least one second radiationdetector, each of said at least one first radiation detector and atleast one second radiation detector capable of receiving radiation fromthe radiation source and producing at least one first output signal fromsaid at least one first radiation detector and at least one secondoutput signal from said at least one second radiation detector; and (b)at least one radiation shield, said radiation shield rotatable about anaxis of shield rotation through an angle of shield rotation, so that amagnitude of said first output signal from said at least one firstradiation detector and a magnitude of said second output signal fromsaid second radiation detector each vary with said angle of shieldrotation.
 22. A sensor according to claim 20, wherein said at least oneradiation shield comprises: (i) a primary radiation shield locatedbetween said at least one first radiation detector and said at least onesecond radiation detector; (ii) at least one first additional radiationshield deployed to interfere with incident radiation directed towardssaid at least one first radiation detector; and (iii) at least onesecond additional radiation shield deployed to interfere with incidentradiation directed towards said at least one second radiation detector.23. The sensor according to claim 22, wherein said at least one firstadditional radiation shield and said at least one second additionalradiation shield are each inclined towards said primary radiationshield.
 24. A sensor according to claim 22, wherein said at least onefirst radiation detector and said at least one second radiation detectorare organized in pairs, each pair having a first member and a secondmember and each radiation shield of said primary and additionalradiation shields is located between one of said first member and one ofsaid second member of one of said pairs so that said output signalvaries with said rotation angle.
 25. The sensor of claim 17,additionally capable of revolving said at least a functional componentabout an axis of revolution through an angle of revolution.
 26. A methodof determining a location of a devise radiation source, the methodcomprising: providing an ionizing radiation source; determining based ondetected radiation from said ionizing radiation source a first plane inwhich said radiation source resides; determining based on said detectedradiation at least a second plane in which said radiation sourceresides; locating said radiation source by calculating an intersectionof said first plane and said at least a second plane.
 27. The method ofclaim 26, wherein determining at least a second plane includesdetermining at least a third plane in which said radiation sourceresides and wherein the method further comprises: calculating a locationof intersection of said first plane, said second plane and said at leasta third plane.
 28. (canceled)
 29. The method of claim 26, wherein saidionizing radiation source has an activity in the range of about 0.01 mCito about 0.5 mCi.
 30. The method of claim 26, wherein said ionizingradiation source has an activity of about 0.1 mCi or less.
 31. Themethod of claim 26, wherein said ionizing radiation source has anactivity of about 0.05 mCi or less.
 32. (canceled)
 33. Use of anionizing radiation source with an activity of 0.1 mCi or less as atarget for non imaging localization or tracking.
 34. A system accordingto claim 1, wherein the processor is configured to determine a planewith respect a center of mass of the source.
 35. A system according toclaim 1, further comprising a displacement mechanism configured to causemotion of at least one of the plurality of detectors, and wherein the atleast one processor is configured to send signals to the displacementmechanism in response to radiation received, to track the radiationsource.
 36. A system according to claim 35, wherein the displacementmechanism is configured to track the radiation source by changinglocations of detection boundaries in order to maintain the source withinthe detection boundaries.
 37. The system of claim 1, further comprisingat least one third radiation detector for producing third signals, andwherein the processor is configured to determine the at least one planeusing the first signals, the second signals, and the third signals. 38.The system of claim 37, further comprising an at least one fourthradiation detector for producing fourth signals, and wherein theprocessor is further configured to determine the at least one planeusing the first signals, the second signals, the third signals and thefourth signals.
 39. The system of claim 37, wherein the at least oneplane is three planes.
 40. The system of claim 38, wherein the at leastone plane is three planes.
 41. The system of claim 1, wherein the sourceincludes a piece of radioactive metal implanted in a body of a subject.42. The system of claim 41, wherein the piece of radioactive metal is awire.